Inclusion Criteria:

* Definite diagnosis of based on compatible chest CT and at least one of the following:

1. biopsy or cytology consistent with LAM, or
2. tuberous sclerosis, renal angiomyolipoma, cystic abdominal lymphangiomas, or chylous effusion in the chest or abdomen, or
3. serum VEGF-D ≥ 800 pg/uL.
* post bronchodilator FEV1 ≤80% predicted or DLCO ≤70% predicted or RV≥120% predicted
* female and postmenopausal status as defined by one of the following:

1. prior bilateral oophorectomy or bilateral ovarian irradiation, or
2. age greater than 55 years, and no menstrual period for 12 months or longer.
3. age 18-55 years and estradiol level in the postmenopausal range in the absence of current use of progestational agents.

* If still premenopausal, may enter if rendered medically postmenopausal on clinical grounds with the use of gonadotropin releasing hormone (e.g. leuprolide), as long as serum estradiol, FSH, and LH are in the postmenopausal range
* Patients with osteopenia or osteoporosis must be receiving appropriate treatment for their osteoporosis or osteopenia at entry into this study.

Exclusion Criteria:

* Known allergy to letrozole
* Inability to comply with pulmonary function tests or follow up visits.
* Treatment with investigational agents within 30 days
* Hormonal therapy (e.g. estrogen, progestin, LHRH agonists or antagonists, estrogen receptor blockers, estrogen receptor down regulators, aromatase inhibitors) within 30 days month of registration
* Medical or psychiatric conditions that would interfere with the ability to provide informed consent.
* abnormal hematologic and hepatic function as defined by the following at the time of randomization.:
* Neutrophils \< 1500/mm3 and platelets \< 100,000/mm3
* Bilirubin \< 1.25 X upper limit of normal
* SGPT (ALT) or SGOT (AST) \>2.5 X upper limit of normal